Dual output variable displacement axial piston pump and method thereof

ABSTRACT

A variable displacement axial pump, including: a housing including first and second inlet ports and first and second outlet ports; a shaft arranged to receive rotational torque; an axis of rotation for the shaft; swash plate located in the housing; a first cylinder block located in the housing and including first and second through-bores and first and second pistons at least partly disposed in the first and second through-bores, respectively, and connected to the swash plate; and a second cylinder block located in the housing and including third and fourth through-bores and third and fourth pistons at least partly disposed in the third and fourth through-bores, respectively, and connected to the swash plate. The shaft is arranged to rotate, with respect to the swash plate, the first and second cylinder blocks about the axis of rotation.

TECHNICAL FIELD

The present disclosure relates to a dual output variable displacement axial piston pump.

BACKGROUND

Known variable displacement axial piston pumps are limited to a single output.

SUMMARY

According to aspects illustrated herein, there is provided a variable displacement axial pump, including: a housing including first and second inlet ports and first and second outlet ports; a shaft arranged to receive rotational torque; an axis of rotation for the shaft; a plate located in the housing; a first cylinder block located in the housing and including first and second through-bores and first and second pistons at least partly disposed in the first and second through-bores, respectively, and connected to the swash plate; and a second cylinder block located in the housing and including third and fourth through-bores and third and fourth pistons at least partly disposed in the third and fourth through-bores, respectively, and connected to the swash plate. The shaft is arranged to rotate, with respect to the swash plate, the first and second cylinder blocks about the axis of rotation.

According to aspects illustrated herein, there is provided a variable displacement axial pump, including: a housing including first and second inlet ports and first and second outlet ports; a shaft arranged to receive rotational torque; an axis of rotation for the shaft; a swash plate located in the housing fixed to prevent rotation with respect to the axis of rotation; a first cylinder block located in the housing, non-rotatably connected to the shaft, and including first and second through-bores and first and second pistons at least partly disposed in the first and second through-bores, respectively, and connected to the swash plate; a second cylinder block located in the housing, non-rotatably connected to the shaft, and including third and fourth through-bores and third and fourth pistons at least partly disposed in the third and fourth through-bores, respectively, and connected to the swash plate; and an actuator arranged to rotate the swash plate about an axis transverse to the axis of rotation. The shaft is arranged to rotate the first and second cylinder blocks. The first cylinder block is arranged to expel first fluid from the first outlet port at a first flow rate. The second cylinder block is arranged to expel second fluid from the second outlet port at a second flow rate.

According to aspects illustrated herein, there is provided a variable displacement axial pump, including: a housing including first and second inlet ports and first and second outlet ports; a shaft arranged to receive rotational torque; an axis of rotation for the shaft; a swash plate located in the housing and fixed to prevent rotation with respect to the shaft; a first cylinder block located in the housing, non-rotatably connected to the shaft, and including first and second through-bores and first and second pistons at least partly disposed in the first and second through-bores, respectively, and connected to the swash plate; a second cylinder block located in the housing, non-rotatably connected to the shaft, and including third and fourth through-bores; and third and fourth pistons at least partly disposed in the third and fourth through-bores, respectively, and connected to the swash plate; and an actuator arranged to rotate the swash plate about an axis transverse to the axis of rotation. For rotation of the shaft about the axis of rotation the swash plate is arranged to displace: the first piston to draw first fluid into the first through-bore via the first inlet port; the second piston to expel second fluid from the second through-bore into the first outlet port; the third piston to draw third fluid into the third through-bore via the second inlet port; and the fourth piston to expel fourth fluid from the fourth through-bore into the second outlet port.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are disclosed, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, in which:

FIG. 1 is a schematic cross-sectional representation of a dual output variable displacement axial piston pump with a rotating swash plate in a first position;

FIG. 2 is schematic cross-sectional representation of the dual output variable displacement axial piston pump in FIG. 1 with cylinder blocks rotated approximately 180 degrees about an axis of rotation;

FIG. 3 is a schematic cross-sectional representation of the dual output variable displacement axial piston pump in FIG. 1 with the rotating swash plate in a second position; and

FIG. 4 is a perspective view of a cylindrical coordinate system demonstrating spatial terminology used in the present application.

DETAILED DESCRIPTION

At the outset, it should be appreciated that like drawing numbers on different drawing views identify identical, or functionally similar, structural elements of the disclosure. It is to be understood that the disclosure as claimed is not limited to the disclosed aspects.

Furthermore, it is understood that this disclosure is not limited to the particular methodology, materials and modifications described and as such may, of course, vary. It is also understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the present disclosure.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. It should be understood that any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure.

FIG. 4 is a perspective view of cylindrical coordinate system 10 demonstrating spatial terminology used in the present application. The present application is at least partially described within the context of a cylindrical coordinate system. System 10 includes axis of rotation, or longitudinal axis, 11, used as the reference for the directional and spatial terms that follow. Opposite axial directions AD1 and AD2 are parallel to axis 11. Radial direction RD1 is orthogonal to axis 11 and away from axis 11. Radial direction RD2 is orthogonal to axis 11 and toward axis 11. Opposite circumferential directions CD1 and CD2 are defined by an endpoint of a particular radius R (orthogonal to axis 11) rotated about axis 11, for example clockwise and counterclockwise, respectively.

To clarify the spatial terminology, objects 12, 13, and 14 are used. As an example, an axial surface, such as surface 15A of object 12, is formed by a plane co-planar with axis 11. However, any planar surface parallel to axis 11 is an axial surface. For example, surface 15B, parallel to axis 11 also is an axial surface. An axial edge is formed by an edge, such as edge 15C, parallel to axis 11. A radial surface, such as surface 16A of object 13, is formed by a plane orthogonal to axis 11 and co-planar with a radius, for example, radius 17A. A radial edge is co-linear with a radius of axis 11. For example, edge 16B is co-linear with radius 17B. Surface 18 of object 14 forms a circumferential, or cylindrical, surface. For example, circumference 19, defined by radius 20, passes through surface 18.

Axial movement is in direction axial direction AD1 or AD2. Radial movement is in radial direction RD1 or RD2. Circumferential, or rotational, movement is in circumferential direction CD1 or CD2. The adverbs “axially,” “radially,” and “circumferentially” refer to movement or orientation parallel to axis 11, orthogonal to axis 11, and about axis 11, respectively. For example, an axially disposed surface or edge extends in direction AD1, a radially disposed surface or edge extends in direction RD1, and a circumferentially disposed surface or edge extends in direction CD1.

FIG. 1 is a schematic cross-sectional representation of dual output variable displacement axial piston pump 100 with a rotating swash plate in a first position. Dual output variable displacement axial pump 100 includes: housing 102; inlet port 104, outlet port 106, inlet port 108, and outlet port 110 in housing 102; shaft 112; axis of rotation AR for shaft 112; swash plate 114 located in housing 102; cylinder blocks 116 and 118 in housing 102; and actuator 120. In an example embodiment, pump 100 includes only one swash plate, for example swash plate 114. By “only one swash plate” we mean that there is no other swash plate in pump 100. In an example embodiment, actuator arm 121 connects actuator 120 to plate 114. Plate 114 is circumferentially fixed with respect to axis of rotation AR. That is, plate 114 does not rotate with shaft 112 about axis AR within housing 102. Port 104 is connected to a fluid reservoir (not shown) providing fluid F1, and port 108 is connected to a fluid reservoir (not shown) providing fluid F2. Inlet ports 104 and 108 can be connected to separate fluid reservoirs or to a common reservoir. Actuator 120 is arranged to pivot, rotate or displace, plate 114 about axis A transverse to axis AR.

Shaft 112 is arranged to rotate blocks 116 and 118 about axis AR and with respect to swash plate 114. Block 116 includes through-bores 122 and 124 and pistons 126 and 128. Pistons 126 and 128 are at least partly disposed in through-bores 122 and 124, respectively, and are engaged with plate 114. By “engaged with” we mean that pistons 126 and 128 maintain connection to plate 114 during rotation of block 116 about axis AR. In an example embodiment, each of pistons 126 and 128 is connected to swash plate 114 via a respective retention assembly 129 as is known in the art.

Block 118 includes through-bores 130 and 132 and pistons 134 and 136. Pistons 134 and 136 are at least partly disposed in through-bores 130 and 132, respectively, and are engaged with plate 114. By “engaged with” we mean that pistons 134 and 136 maintain connection to plate 114 during rotation of block 118 about axis AR. In an example embodiment, each of pistons 134 and 136 is connected to swash plate 114 via a respective retention assembly 129, as is known in the art.

For rotation of block 116 about axis AR: pistons 126 and 128 are arranged to draw fluid F1, via port 104, into through-bores 122 and 124, respectively; and expel fluid F1 from through-bores 122 and 124 into port 106 at a flow rate, or pump rate. For rotation of block 118 about axis AR: pistons 134 and 136 are arranged to draw fluid F2, via port 108, into through-bores 130 and 132, respectively; and expel fluid F2 from through-bores 130 and 132 into port 110 at a flow rate, or pump rate. In the example of FIG. 1, the flow rates at ports 106 and 110 are 137A and 137B, respectively.

Flow rate 137A is dependent upon the speed of rotation of block 116 and the displacement of pistons 126 and 128, by plate 114, within through-bores 122 and 124, respectively. The speed of rotation of block 116 is a function of engine E for a vehicle or device (not shown) housing pump 100 and is determined by operations of the vehicle or device other than that for pump 100. That is, the speed of rotation is not controllable by pump 100. For a given position of plate 114 about axis A, increasing or decreasing the speed of rotation of block 116 increases or decreases rate 137A, respectively.

Flow rate 137B is dependent upon the speed of rotation of block 118 and the displacement of pistons 134 and 136, by plate 114, within through-bores 130 and 132, respectively. The speed of rotation of block 118 is a function of engine E. For a given position of plate 114 about axis A, increasing or decreasing the speed of rotation of block 118 increases or decreases rate 137B, respectively.

FIG. 2 is a schematic cross-sectional representation of dual port variable displacement axial piston pump 100 in FIG. 1 with cylinder blocks 116 and 118 rotated approximately 180 degrees about axis of rotation AR. In the example of FIG. 2, as a result of the rotation of blocks 116 and 118 180 degrees about axis AR: through-bores 122, 124, 130, and 132 are aligned, in direction AD1, with ports 106, 104, 110, and 108, respectively. Plate 114 has simultaneously:

1. Displaced piston 126 in axial direction AD2, within through-bore 122.

2. Displaced piston 128 in axial direction AD1 within through-bore 124.

3. Displaced piston 134 in axial direction AD1, within through-bore 130.

4. Displaced piston 136 in axial direction AD2 within through-bore 132.

As a result of the rotation of blocks 116 and 118 in FIG. 2: fluid F1 in through-bore 122 (drawn into through-bore 122 in FIG. 1) is expelled into port 106 by displacement of piston 126 in direction AD2; fluid F1 is drawn into through-bore 124 due to suction produced by displacement of piston 128 in direction AD1; fluid F2 in through-bore 130 (drawn into through-bore 130 in FIG. 1) is expelled into port 110 by displacement of piston 134 in direction AD1; and fluid F2 is drawn into through-bore 132 by suction produced by displacement of piston 136 in direction AD2.

The continued rotation of blocks 116 and 118 about axis AR results in a repeating cycle of: drawing fluid F1 from port 104 into through-bores 122 and 124, and expelling fluid F1 from through-bores 122 and 124 into port 106; and, drawing fluid F2 from port 108 into through-bores 130 and 132, and expelling fluid F2 from through-bores 130 and 132 into port 110. The preceding cycle is applicable to any rotational speed of shaft 112 and any circumferential position of plate 114 with respect to axis A. With the configuration of ports 104, 106, 108 and 110 shown in FIG. 1, angle 139 between plate 114 and axis AR is acute (less than 90 degrees). For a configuration (not shown) in which the respective positions of ports 104 and 106 are switched and the respective positions of ports 108 and 110 are switched, angle 139 between plate 114 and axis AR is obtuse (greater than 90 degrees).

Flow rates 137A and 137B are governed by the circumferential position of plate 114 with respect to axis A. The circumferential position of plate 114 determines the distance that pistons 126, 128, 134, and 136 are displaced by plate 114 within through-bores 122, 124, 130, and 132, respectively. As seen in the example of FIGS. 1 and 2: piston 126 displaces distance 138 in direction AD2 in the transition from FIG. 1 to FIG. 2; and piston 128 displaces distance 138 in direction AD1 in the transition from FIG. 2 to FIG. 1. Thus, pistons 126 and 128 displace distance 138 to draw fluid F1 into through-bores 122 and 124, respectively, and pistons 126 and 128 displace distance 138 to expel fluid F1 from through-bores 122 and 124, respectively.

As discussed below, changing the extent of the axial displacement of pistons 126 and 128 (for example, distance 138) changes the amount of fluid F1 drawn into and expelled by block 116 and hence changes flow rate 137A.

As seen in the example of FIGS. 1 and 2: piston 134 displaces distance 140 in direction AD1 in the transition from FIG. 1 to FIG. 2; and piston 136 displaces distance 140 in direction AD2 in the transition from FIG. 2 to FIG. 1. Thus, pistons 134 and 136 displace distance 140 to draw fluid F2 into through-bores 130 and 132, respectively, and pistons 134 and 136 displace distance 140 to expel fluid F2 from through-bores 130 and 132, respectively.

As discussed below, changing the extent of the axial displacement of pistons 134 and 136 (for example, distance 140) changes the amount of fluid F2 drawn into and expelled by block 118 and hence changes flow rate 137B.

FIG. 3 is a schematic cross-sectional representation of dual output variable displacement axial piston pump 100 in FIG. 1 with rotating swash plate 114 in a second circumferential position. In the example of FIG. 3, swash plate 114 has been rotated, by actuator 120, in direction CD1 about axis A from the position shown in FIG. 1. The alignment of through-bores 122, 124, 130, and 132 with ports 104, 106, 108, and 110 remains the same. The rotation and subsequent tilting of swash plate 114 in FIG. 3 changes the axial displacement of pistons 126, 128, 134, and 136. As a result: pistons 126 and 128 are displaced distance 142 by plate 114; and pistons 134 and 136 are displaced distance 144 by plate 114. In the example of FIGS. 1 through 3: distance 142 is less than distance 138; and distance 144 is less than distance 140.

Assuming a constant speed of rotation of shaft 112 in FIGS. 1 through 3, and since distance 142 is less than distance 138, the amount of fluid F1 drawn into and expelled from through-bores 122 and 124 in FIG. 3 (flow rate 137C) decreases in comparison to the amount of fluid F1 drawn into through-bores 122 and 124 in FIGS. 1 and 2. Assuming the constant speed of rotation of shaft 112 in FIGS. 1 through 3, and since distance 144 is less than distance 140, the amount of fluid F2 drawn into and expelled from through-bores 130 and 132 in FIG. 3 (flow rate 137D) decreases in comparison to the amount of fluid F2 drawn into through-bores 130 and 132 in FIGS. 1 and 2. Thus rates 137C and 137D in FIG. 3 are less than rates 137A and 137B, respectively, in FIGS. 1 and 2.

The following discussion is directed to a transition from FIG. 3 to FIG. 1. To transition from FIG. 3 to FIG. 1, swash plate 114 is pivoted, by actuator 120, about axis A in direction CD1. Assuming the constant speed of rotation of shaft 112 in FIGS. 1 through 3, and since distance 142 is less than distance 138, the amount of fluid F1 drawn into and expelled from through-bores 122 and 124 in FIG. 1 increases in comparison to the amount of fluid F1 drawn into through-bores 122 and 124 in FIG. 3. Assuming the constant speed of rotation of shaft 112 in FIGS. 1 through 3, and since distance 144 is less than distance 140, the amount of fluid F2 drawn into and expelled from through-bores 130 and 132 in FIG. 1 increases in comparison to the amount of fluid F2 drawn into and expelled from through-bores 130 and 132 in FIG. 3. Thus rates 137A and 137B in FIGS. 1 and 2 are greater than rates 137C and 137D, respectively, in FIG. 3.

The following should be viewed in light of FIGS. 1 through 3. The following describes a method using variable displacement axial pump 100. A first step positions, using actuator 120, swash plate 114 in a first circumferential position with respect to axis A. A second step rotates, with engine E, shaft 112 about axis AR. A third step rotates cylinder blocks 116 and 118 about axis AR and with respect to swash plate 114. A fourth step axially displaces, with plate 114, pistons 126, 128, 134 and 136. A fifth step draws fluid F1, via inlet port 104 and using pistons 126 and 128, into through-bores 122 and 124, respectively. A sixth step expels, using pistons 126 and 128, fluid F1 from through-bores 122 and 124, respectively, into outlet port 106. A seventh step draws fluid F2, via inlet port 108 and using pistons 134 and 136, into through-bores 130 and 132, respectively. An eighth step expels, using pistons 134 and 136, fluid F2 from through-bores 130 and 132, respectively, into outlet port 110.

A ninth step aligns, in axial direction AD1, through-bores 122, 124, 130 and 132 with port 104, port 106, port 108 and port 110, respectively. Axially displacing, with plate 114, pistons 126, 128, 134 and 136 includes simultaneously displacing, with swash plate 114: piston 126 distance 138, in axial direction AD1, within through-bore 122; piston 128 distance 138, in axial direction AD2, within through-bore 124; piston 134 distance 140, in axial direction AD2, within through-bore 130; and piston 136 distance 140, in axial direction AD1, within through-bore 132.

A tenth step pivots, with actuator 120, plate 114 in direction CD2 about axis A. An eleventh step aligns, in axial direction AD1, through-bores 122, 124, 130 and 132 with port 104, port 106, port 108 and port 110, respectively. Axially displacing, with plate 114, pistons 126, 128, 134 and 136 includes simultaneously displacing, with swash plate 114: piston 126 distance 142, in axial direction AD1, within through-bore 122; piston 128 distance 142, in axial direction AD2, within through-bore 124; piston 134 distance 144, in axial direction AD2, within through-bore 130; and piston 136 distance 144, in axial direction AD1, within through-bore 132.

A twelfth step pivots plate 114 in direction CD1 about axis A. A thirteenth step aligns, in axial direction AD1, through-bores 122, 124, 130 and 132 with port 104, port 106, port 108 and port 110, respectively. Axially displacing, with plate 114, pistons 126, 128, 134 and 136 includes simultaneously displacing, with swash plate 114: piston 126 distance 138, in axial direction AD1, within through-bore 122; piston 128 distance 138, in axial direction AD2, within through-bore 124; piston 134 distance 140, in axial direction AD2, within through-bore 130; and piston 136 distance 140, in axial direction AD1, within through-bore 132.

The following should be viewed in light of FIGS. 1 through 3. The following describes a method using variable displacement axial pump 100. A first step positions, using actuator 120, swash plate 114 in a first circumferential position with respect to axis A. A second step rotates, with engine E, shaft 112 about axis AR. A third step rotates cylinder blocks 116 and 118 about axis AR and with respect to swash plate 114. A fourth step, for a constant speed of rotation of shaft 112: displaces, with swash plate 114, pistons 126 and 128; draws fluid F1, with pistons 126 and 128 and from inlet port 104, into cylinder block 116; expels, with pistons 126 and 128 and from cylinder block 116, fluid F1 into outlet port 106 at flow rate 137A; displaces, with swash plate 114, pistons 134 and 136; draws fluid F2, with pistons 134 and 136 and from inlet port 108, into cylinder block 118; expels, with pistons 134 and 136 and from cylinder block 118, fluid F2 into outlet port 110 at flow rate 137B.

A fifth step pivots, using actuator 120, swash plate 114 in circumferential direction CD1 with respect to axis A; A sixth step, for the constant speed of rotation of shaft 112: displaces, with swash plate 114, pistons 126 and 128; draws fluid F1, with pistons 126 and 128 and from inlet port 104, into cylinder block 116; expels, with pistons 126 and 128 and from cylinder block 116, fluid F1 into outlet port 106 at flow rate 137C, less than the flow rate 137A; displaces, with swash plate 114, pistons 134 and 136; draws fluid F2, with pistons 134 and 136 and from inlet port 108, into cylinder block 118; expels, with pistons 134 and 136 and from cylinder block 118, fluid F2 into outlet port 110 at flow rate 137D, less than flow rate 137B.

A seventh step pivots, using actuator 120 and from the first circumferential position of swash plate 114, swash plate 114 in circumferential direction CD2. A eighth step, for the constant speed of rotation of shaft 112: displaces, with swash plate 114, pistons 126 and 128; draws fluid F1, with pistons 126 and 128 and from inlet port 104, into cylinder block 116; expels, with pistons 126 and 128 and from cylinder block 116, fluid F1 into outlet port 106 at flow rate 137A, greater than flow rate 137C; displaces, with swash plate 114, pistons 134 and 136; draws fluid F2, with pistons 134 and 136 and from inlet port 108, into cylinder block 118; and expels, with pistons 134 and 136 and from cylinder block 118, fluid F2 into outlet port 110 at flow rate 137B, greater than flow rate 137D.

The following should be viewed in light of FIGS. 1 through 3. The following describes a method using variable displacement axial pump 100. A first step rotates shaft 112 about axis AR. A second step rotates cylinder blocks 116 and 118, about axis AR and with respect to swash plate 114. A third step simultaneously: draws, with piston 126, fluid into through-bore 122 via inlet port 104; expels, with piston 128, fluid from through-bore 124 into outlet port 106; draws, with piston 134, fluid into through-bore 130 via inlet port 108; and expels, with piston 136, fluid from through-bore 132 into outlet port 110.

Advantageously, pump 100 simultaneously implements two pumping operations (for example, fluid F1 through port 104 to port 106, and fluid F2 through port 108 to port 110) using a single swash plate 114. By using single swash plate 114 and single actuator 120 to control flow from cylinder blocks 116 and 118, and hence produce two fluid streams, a second swash plate and actuator, normally needed to produce two fluid streams, can be eliminated, reducing the overall space requirement, component count, and cost requirements for producing two fluid streams.

It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

LIST OF REFERENCE CHARACTERS

-   10 cylindrical system -   11 axis of rotation -   AD1 axial direction -   AD2 axial direction -   RD1 radial direction -   RD2 radial direction -   CD1 circumferential direction about axis AR -   CD2 circumferential direction about axis AR -   R radius -   12 object -   13 object -   14 object -   15A surface -   15B surface -   15C edge -   16A surface -   16B edge -   17A radius -   17B radius -   18 surface -   19 circumference -   20 radius -   A axis transverse to axis AR -   AR axis of rotation for shaft 112 -   CD1 circumferential direction about axis A -   CD2 circumferential direction about axis A -   100 variable displacement axial pump -   102 housing -   104 inlet port -   106 inlet port -   108 outlet port -   110 outlet port -   112 shaft -   114 swash plate -   116 cylinder block -   118 cylinder block -   120 actuator -   121 actuator arm -   122 through-bore in block 116 -   124 through-bore in block 116 -   126 piston in block 116 -   128 piston in block 116 -   129 retention assembly -   130 through-bore in block 118 -   132 through-bore in block 118 -   134 piston in block 118 -   136 piston in block 118 -   137A flow rate for block 116 -   137B flow rate for block 118 -   137C flow rate for block 116 -   137D flow rate for block 118 -   138 displacement distance for pistons 126 and 128 -   139 acute angle between shaft 112 and swash plate 114 -   140 displacement distance for pistons 134 and 136 -   142 displacement distance for pistons 126 and 128 -   144 displacement distance for pistons 134 and 136 

1. A variable displacement axial pump, comprising: a housing including: first and second inlet ports; and, first and second outlet ports; a shaft arranged to receive rotational torque; an axis of rotation for the shaft; a swash plate located in the housing; a first cylinder block located in the housing and including: first and second through-bores; and, first and second pistons at least partly disposed in the first and second through-bores, respectively, and connected to the swash plate; and, a second cylinder block located in the housing and including: third and fourth through-bores; and, third and fourth pistons at least partly disposed in the third and fourth through-bores, respectively, and connected to the swash plate, wherein the shaft is arranged to rotate, with respect to the swash plate, the first and second cylinder blocks about the axis of rotation.
 2. The variable displacement axial pump of claim 1, wherein: the first and second pistons are arranged to: draw first fluid, via the first inlet port, into the first and second through-bores, respectively; and, expel the first fluid from the first and second through-bores, respectively, into the first outlet port; and, the third and fourth pistons are arranged to: draw second fluid, via the second inlet port, into the third and fourth through-bores, respectively; and, expel the second fluid from the third and fourth through-bores, respectively, into the second outlet port.
 3. The variable displacement axial pump of claim 2, further comprising: an actuator arranged to pivot the swash plate about an axis transverse to the axis of rotation, wherein in a first circumferential position of the swash plate, with respect to the axis, and at a constant rotational speed for the shaft: the first cylinder block is arranged to expel the first fluid into the first outlet port at a first flow rate; the second cylinder block is arranged to expel the second fluid into the second outlet port at a second flow rate; and, the first and second flow rates are equal.
 4. The variable displacement axial pump of claim 3, wherein in a second circumferential position of the swash plate, different from the first circumferential position of the swash plate, with respect to the axis, and at the constant rotational speed for the shaft: the first cylinder block is arranged to expel the first fluid into the first outlet port at a third flow rate different from the first flow rate; and, the second cylinder block is arranged to expel the second fluid into the second outlet port at a fourth flow rate different from the second flow rate.
 5. The variable displacement axial pump of claim 2, further comprising: an actuator arranged to pivot the swash plate about an axis transverse to the axis of rotation, wherein in a first circumferential position of the swash plate, with respect to the axis, and at a constant rotational speed for the shaft: the first cylinder block is arranged to expel the first fluid into the first outlet port at a first flow rate; the second cylinder block is arranged to expel the second fluid into the second outlet port at a second flow rate; and, the first and second flow rates are not equal.
 6. The variable displacement axial pump of claim 5, wherein in a second circumferential position of the swash plate, different from the first circumferential position of the swash plate, with respect to the axis, and at the constant rotational speed for the shaft: the first cylinder block is arranged to expel the first fluid into the first outlet port at a third flow rate different from the first flow rate; and, the second cylinder block is arranged to expel the second fluid into the second outlet port at a fourth flow rate different from the second flow rate.
 7. The variable displacement axial pump of claim 1, further comprising: an actuator arranged to pivot the swash plate about an axis transverse to the axis of rotation, wherein in a first circumferential position of the swash plate, with respect to the axis and with the first, second, third and fourth through-bores aligned with the first, second, third and fourth ports, respectively, the swash plate is arranged to simultaneously: displace the first piston in a first axial direction by a first distance; displace the second piston in a second axial direction, opposite the first axial direction, by the first distance; displace the third piston in the second axial direction by a second distance; and, displace the fourth piston in the first axial direction by the second distance.
 8. The variable displacement axial pump of claim 7, wherein in a second circumferential position of the swash plate, with respect to the axis, different from the first circumferential position of the swash plate and with the first, second, third and fourth through-bores aligned with the first, second, third and fourth ports, respectively, the swash plate is arranged to simultaneously: displace the first piston in a first axial direction by a third distance, different from the first distance; displace the second piston in the second axial direction, by the third distance; displace the third piston in the second axial direction by a fourth distance, different from the second distance; and, displace the fourth piston in the first axial direction by the fourth distance.
 9. The variable displacement axial pump of claim 1, further comprising: an actuator arranged to pivot the swash plate about an axis transverse to the axis of rotation, wherein pivoting, with the actuator, the swash plate in a first circumferential direction about the axis is arranged to simultaneously: increase displacement, in a first axial direction, of the first piston within the first through-bore; increase displacement, in a second axial direction opposite the first axial direction, of the second piston in the second through-bore; increase displacement, in the second axial direction, of the third piston in the third through-bore; and, increase displacement, in the first axial direction, of the fourth piston in the fourth through-bore.
 10. The variable displacement axial pump of claim 9, wherein pivoting, with the actuator, the swash plate in a second circumferential direction, opposite the first circumferential direction, about the axis is arranged to simultaneously: decrease displacement, in the first axial direction, of the first piston within the first through-bore; decrease displacement, in the second axial direction, of the second piston in the second through-bore; decrease displacement, in the second axial direction, of the third piston in the third through-bore; and, decrease displacement, in the first axial direction, of the fourth piston in the fourth through-bore.
 11. The variable displacement axial pump of claim 1, wherein the swash plate is an only swash plate for the variable displacement axial pump.
 12. A variable displacement axial pump, comprising: a housing including: first and second inlet ports; and, first and second outlet ports; a shaft arranged to receive rotational torque; an axis of rotation for the shaft; only one swash plate located in the housing fixed to prevent rotation with respect to the axis of rotation; a first cylinder block located in the housing, non-rotatably connected to the shaft, and including: first and second through-bores; and, first and second pistons at least partly disposed in the first and second through-bores, respectively, and connected to the swash plate; a second cylinder block located in the housing, non-rotatably connected to the shaft, and including: third and fourth through-bores; and, third and fourth pistons at least partly disposed in the third and fourth through-bores, respectively, and connected to the swash plate; and, an actuator arranged to rotate the swash plate about an axis transverse to the axis of rotation, wherein: the shaft is arranged to rotate the first and second cylinder blocks; the first cylinder block is arranged to expel first fluid from the first outlet port at a first flow rate; and, the second cylinder block is arranged to expel second fluid from the second outlet port at a second flow rate.
 13. A variable displacement axial pump, comprising: a housing including: first and second inlet ports; and, first and second outlet ports; a shaft arranged to receive rotational torque; an axis of rotation for the shaft; only one swash plate located in the housing and fixed to prevent rotation with respect to the shaft; a first cylinder block located in the housing, non-rotatably connected to the shaft, and including: first and second through-bores; and, first and second pistons at least partly disposed in the first and second through-bores, respectively, and connected to the swash plate; a second cylinder block located in the housing, non-rotatably connected to the shaft, and including: third and fourth through-bores; and, third and fourth pistons at least partly disposed in the third and fourth through-bores, respectively, and connected to the swash plate; and, an actuator arranged to rotate the swash plate about an axis transverse to the axis of rotation, wherein for rotation of the shaft about the axis of rotation the swash plate is arranged to displace: the first piston to draw first fluid into the first through-bore via the first inlet port; the second piston to expel second fluid from the second through-bore into the first outlet port; the third piston to draw third fluid into the third through-bore via the second inlet port; and, the fourth piston to expel fourth fluid from the fourth through-bore into the second outlet port.
 14. A method of using the variable displacement axial pump of claim 1, comprising: rotating the shaft about the axis of rotation; rotating, with the shaft, the first and second cylinder blocks about the axis of rotation and with respect to the swash plate; axially displacing, with the swash plate, the first, second, third and fourth pistons; drawing first fluid, via the first inlet port and using the first and second pistons, into the first and second through-bores, respectively; expelling, using the first and second pistons, the first fluid from the first and second through-bores, respectively, into the first outlet port; drawing second fluid, via the second inlet port and using the third and fourth pistons, into the third and fourth through-bores, respectively; and, expelling, using the third and fourth pistons, the second fluid from the third and fourth through-bores, respectively, into the second outlet port.
 15. The method of claim 14, further comprising: aligning, in a first axial direction, the first, second, third and fourth through-bores with the first inlet port, the first outlet port, the second inlet port and the second outlet port, respectively; and, simultaneously displacing, with the swash plate: the first piston a first distance, in the first axial direction, within the first through-bore; the second piston the first distance, in a second axial direction opposite the first axial direction, within the second through-bore; the third piston a second distance, in the second axial direction, within the third through-bore; and, the fourth piston the second distance, in the first axial direction, within the fourth through-bore.
 16. The method of claim 15, further comprising: pivoting, using the actuator, the swash plate about the axis; aligning, in the first axial direction, the first, second, third and fourth through-bores with the first inlet port, the first outlet port, the second inlet port and the second outlet port, respectively; and, simultaneously displacing, with the swash plate: the first piston a third distance, in the first axial direction, within the first through-bore; the second piston the third distance, in the second axial direction, within the second through-bore; the third piston a fourth distance, in the second axial direction, within the third through-bore; and, the fourth piston the fourth distance, in the first axial direction, within the fourth through-bore, wherein: the third distance is different from the first distance; and, the fourth distance is different from the second distance.
 17. A method of using the variable displacement axial pump of claim 12, comprising: rotating the shaft about the axis of rotation at a constant speed; rotating, with the shaft, the first and second cylinder blocks about the axis of rotation; and, for a constant speed of rotation of the shaft: displacing, with the swash plate, the first and second pistons; drawing first fluid, with the first and second pistons and from the first inlet port, into the first cylinder block; expelling, with the first and second pistons and from the first cylinder block, the first fluid into the first outlet port at a first flow rate; displacing, with the swash plate, the third and fourth pistons; drawing second fluid, with the third and fourth pistons and from the second inlet port, into the second cylinder block; and, expelling, with the third and fourth pistons and from the second cylinder block, the second fluid into the second outlet port at a second flow rate.
 18. The method of claim 17, further comprising: pivoting, using the actuator, the swash plate in a first circumferential direction with respect to the axis; and, for the constant speed of rotation of the shaft: displacing, with the swash plate, the first and second pistons; drawing the first fluid, with the first and second pistons and from the first inlet port, into the first cylinder block; expelling, with the first and second pistons and from the first cylinder block, the first fluid into the first outlet port at a third flow rate, less than the first flow rate; displacing, with the swash plate, the third and fourth pistons; drawing the second fluid, with the third and fourth pistons and from the second inlet port, into the second cylinder block; and, expelling, with the third and fourth pistons and from the second cylinder block, the second fluid into the second outlet port at a fourth flow rate, less than the second flow rate.
 19. The method of claim 18, further comprising: pivoting, using the actuator, the swash plate in a second circumferential direction, opposite the first circumferential direction; and, for the constant speed of rotation of the shaft: displacing, with the swash plate, the first and second pistons; drawing the first fluid, with the first and second pistons and from the first inlet port, into the first cylinder block; expelling, with the first and second pistons and from the first cylinder block, the first fluid into the first outlet port at a fifth flow rate, greater than the third flow rate; displacing, with the swash plate, the third and fourth pistons; drawing the second fluid, with the third and fourth pistons and from the second inlet port, into the second cylinder block; and, expelling, with the third and fourth pistons and from the second cylinder block, the second fluid into the second outlet port at a sixth flow rate, greater than the fourth flow rate.
 20. A method of using the variable displacement axial pump of claim 13, comprising: rotating the shaft about the axis of rotation; and, rotating, with the shaft, the first and second cylinder blocks about the axis of rotation; displacing, with the swash plate, the first, second, third and fourth pistons; and, simultaneously: drawing, with the first piston, first fluid into the first through-bore via the first inlet port; expelling, with the second piston, second fluid from the second through-bore into the first outlet port; drawing, with the third piston, third fluid into the third through-bore via the first inlet port; and, expelling, with the fourth piston, fourth fluid from the fourth through-bore into the second outlet port. 